New Zealand Journal of Geology and Geophysics abstracts
The October 1999 Mt Adams rock avalanche and subsequent landslide
dam-break flood and effects in Poerua River, Westland, New Zealand
Graham T. Hancox
Mauri J. McSaveney
Vernon R. Manville
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
Tim R. Davies
Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
Abstract On 6 October 1999 a very large
(c. 10–15 million m3) rock avalanche from Mt Adams
blocked the Poerua River 11 km upstream from the SH6 road bridge
on the West Coast of the South Island. The 120 m high rock debris
dam impounded a lake with a volume of 5–7 million m3 before
it overtopped on 7 October. The short survival time of landslide dams
in rivers in Westland and around the world suggested a high probability
of rapid dam failure and flooding downstream. This was confirmed when
the dam breached 6 days later on 12 October 1999, during the first
significant rain after the landslide occurred.
The resulting dam-break flood deposited considerable
coarse gravel from the landslide in the valley downstream of the dam,
and (mostly fines) on the alluvial fan below the Poerua gorge exit. The
flow inundated farmland in the upper Poerua valley, but otherwise was
largely confined to the river channel and did little damage at the
time, mainly because of significant flow attenuation (c. 50% or
greater) and sediment deposition on the alluvial fan below the gorge
exit. Subsequently the remnant lake has been infilled, and c. 75% of
the dam material has been transported downstream during floods. About
1.7 million m3 of alluvium has been deposited in the river
channel and across farmland between the gorge exit and the SH6 bridge,
changing the river's course and causing lateral erosion of older
terraces. These effects are continuing and will cause ongoing problems
in the future.
The rock avalanche and landslide dam failure in the Poerua
valley were significant events which have had a profound local
geomorphic impact. The landslide and subsequent downstream effects are
typical landscape-forming events. These events and the resulting
community response to them have provided valuable information on the
hazards, effects, and management of future landslide-dam failures in
Westland. Increased resources within local authorities for hazard
assessments and response planning during such events would reduce the
risk from dam-break floods in the future, especially following the next
Alpine Fault earthquake, when further landslide dam failures are
expected.
Keywords rock avalanche; landslide dam; dam-break
flood; Mt Adams; Poerua River; Southern Alps; Westland; landslide
hazard mitigation
G04046; Received 1 December 2004; accepted 19 August 2005; Online
publication date 13 October 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
683–705
0028–8306/05/4804–0683 © The Royal Society of New Zealand 2005
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